1. Field of the Invention
The present invention involves is a method and a circuit arrangement for the processing of a signal in connection with its reception, when the signal conforms to one or more system specifications. The invention is preferably applied in receivers operating on two frequency ranges or in receivers intended to be used in two radio communication systems. Preferably the receiver is a part of a mobile station or a base station of a cellular system.
2. Description of the Prior Art
Mobile communication systems develop and grow very fast, so that in many regions systems according to several different standards have been constructed or are under construction. Therefore there is now a need for such mobile stations and base stations which can be used in more than one system. For instance in the USA, Direct Sequence Spread Spectrum (DSSS) systems will be built in addition to the systems based on the frequency modulation (FM) technology now in use. Characteristics of receivers operating in these two system types are specified in the standard IS-95.
There are further in development so called third generation systems, which probably will require a dual-mode operation of the receiver. Of these systems we could mention the Universal Mobile Telecommunications System (UMTS) defined by the European Telecommunications Standards Institute (ETSI), and the Future Public Land Mobile Telecommunications Systems (FPLMTS) defined by the radio suction of the International Telecommunication Union.
A receiver operating in two systems must be able to process signals conforming to two different specifications. The signals of these systems may have different bit rates, signal bandwidths, and channel rasters. Channel raster means the frequency difference between two adjacent channels. For instance in the GSM system the signal bit rate is 270.833 kbit/s, the channel raster is 200 kHz, and the signal bandwidth is 200 kHz. On the other hand, for instance in the DECT system the bit rate is 1.152 Mbit/s, the channel raster is 1728 kHz and the signal bandwidth is about 1 MHz. In future systems, such as in e.g. the UMTS system, the bandwidth required for the transmission of a signal will probably be wider than in the GSM and DECT systems, due to the higher bit rate.
FIG. 1a shows a previously known circuit arrangement 100 for the reception of signals of two systems. The radio frequency (RF) signal Rx received via the antenna 101 is supplied into two receive branches A and B. The signal according to the system specification A is filtered in the band-pass filter 102a, amplified 104a and mixed to the intermediate frequency by the signal obtained from the oscillator 108a, so that the output of the mixer 106a provides the intermediate frequency signal IF1. The intermediate frequency signal is further supplied to the band-pass filter 110a, and the signal obtained from this is amplified 112a. Then this signal is mixed by a signal obtained from a second oscillator 116a, whereby the output of the mixer 114a provides a second intermediate frequency signal IF2. The second intermediate frequency signal is band-pass filtered 118a and amplified 120a.
Correspondingly, a signal according to the second system specification B is processed in the second signal branch B, which comprises units corresponding to those of the signal branch A: a band-pass filter 102b, an amplifier 104b, and a mixer 106b with an accompanying oscillator 108b, and a band-pass filter 110b, an amplifier 112b, and a mixer 114b with an accompanying oscillator 116b, and a band-pass filter 118b and an amplifier 120b.
Due to the band-pass filters the signal according to the system specification A is prevented to propagate in the signal branch B, and the signal according to the system specification B is prevented to propagate in the signal branch A. The signal branch can be selected with a switch (not shown in FIG. 1) if the systems A and B use the same RF frequency band.
The switch 122 selects either the signal from the signal branch A or the signal from the signal branch B to be supplied to the input of the analog-to-digital converter 124. In the analog-to-digital converter 124 the signal is sampled into digital samples by a sampling frequency determined by the sampling signal f .sub.s. The signal converted into digital samples is further supplied to the digital signal processing unit 126, which performs signal detection and provides a digital baseband output signal, which further can be converted into an analog signal by a digital-to-analog converter (not shown in FIG. 1a).
For instance, in a mobile communications network the signal selected for the analog-to-digital converter can be controlled in accordance with a change-over instruction transmitted by the mobile network. Then the signal processing unit detects this instruction and controls the switch 122 in the manner indicated by the instruction. If the mobile system permits the user of a mobile station to select the system he uses, then the processor of the mobile station generates a control instruction to the switch, based on an instruction given by the user via the a interface, such as a keyboard. FIG. 1b shows the pass-band of the band-pass filter 102a, which is dimensioned according to the frequency range used in the system A. The filter 102a has a pass-band width B11a and a center frequency F11a. Correspondingly, FIG. 1c shows the pass-band of the band-pass filter 102b, which is dimensioned according to the frequency range used in the system B. The filter 102b has a pass-band width B11b and a center frequency F11b.
FIG. 1d shows the pass-band of the first intermediate frequency band-pass filter 110a in the signal branch A. The width of this pass-band is B12a and the center frequency F12a. Correspondingly, FIG. 1e shows the pass-band of the first intermediate frequency band-pass filter 110b in the signal branch B. The width of this pass-band is B12b and the center frequency F12b. Further, the FIG. 1f shows the pass-band of the second intermediate frequency band-pass filter 118a in the signal branch A, having a pass-band width B13a and the center frequency F13a. Correspondingly, the FIG. 1g shows the pass-band of the second intermediate frequency band-pass filter 118b in the signal branch B, having a pass-band width B13b and the center frequency F13b. FIGS. 1d, 1e, 1f and 1g show that the pass-bands of the intermediate frequency band-pass filters are narrower in the signal branch A and wider in the signal branch B, according to the specifications of the systems A and B.
The most significant disadvantage of the prior art presented above is that it requires the use of separate intermediate frequency band-pass filters for each system. These filters must have a high attenuation outside the signal band, and therefore they are large-sized and expensive components. Due to the two intermediate frequency signal branches also the number of mixer and amplifier components is high. Alternatively the mixer and amplifier components can also be used for both signals, but this requires a substantially high number of controllable switches, with which the signal is connected to the intermediate frequency filters according to the respective system.